I have been looking at the postings in the classifieds for a used SCT. But, one question always comes to my mind as I consider buying a used scope: "How durable are the coatings used on the correctors and on the mirrors?" Is it safe to buy a 20 year old scope? a 10 year old scope? or, should I stick with something newer? How long can I expect the coatings to last? If I bought a new SCT, how long would those coatings last?
Durability of Coatings
Started by psimone, 12/05/2004 12:43AM
Posted 12/05/2004 12:43AM
Opening Post
Posted 12/05/2004 03:41AM
| Edited 12/05/2004 03:43AM
#1
Paul:
I think you have to consider where you're buying it. Several years ago I sold a 8" Meade LX200 on Ebay. I had lived at the coast for a couple of years and I didn't realize that the coating had been deteriorated by the salt air.
So, don't buy a used scope from any place that is close to the ocean.
As for the coatings on the commercial scopes they generally do it every two to three years. I've visited the Naval observatory at Flagstaff and work at Mt. Hopkins.
The VERITAS telescope anodizes there mirrors but they are out in the open with no covering and anodizing protects them for some time. It's a simple procedure and takes a very short length of time but I doubt if it's done on the amateur scopes.
The anodization does effect the reflectivity somewhat but the benefits far exceed the minor loss at the high frequency end of the spectrum (above 450)
Darrell
I think you have to consider where you're buying it. Several years ago I sold a 8" Meade LX200 on Ebay. I had lived at the coast for a couple of years and I didn't realize that the coating had been deteriorated by the salt air.
So, don't buy a used scope from any place that is close to the ocean.
As for the coatings on the commercial scopes they generally do it every two to three years. I've visited the Naval observatory at Flagstaff and work at Mt. Hopkins.
The VERITAS telescope anodizes there mirrors but they are out in the open with no covering and anodizing protects them for some time. It's a simple procedure and takes a very short length of time but I doubt if it's done on the amateur scopes.
The anodization does effect the reflectivity somewhat but the benefits far exceed the minor loss at the high frequency end of the spectrum (above 450)
Darrell
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
Posted 12/05/2004 05:11AM
#2
This is my view:
1. My understanding is that SCT optics have improved over the years, and the newer ones are consistently better than the older ones. SCT's manufactured during high demand comet crazes seem to be of low quality.
2. Check out what Rod Mollise has to say about buying a used SCT on his website, members.aol.com/rmollise Rod is the Guru on SCTs and he literally wrote the book, Choosing and Using the SCT....
3. Coatings have also steadly improved over the years.
4. The mounts, drives etc have steadly improved over the years, those oldies, not so fun to use....
5. Since this is the beginners forum, I am assuming this is your first scope. If so, I hope you have tried out some various SCTs and understand the differences in the mounts and the various problems associated with them...
Otherwise, a trip to a star party or club outing is in order...
jon
1. My understanding is that SCT optics have improved over the years, and the newer ones are consistently better than the older ones. SCT's manufactured during high demand comet crazes seem to be of low quality.
2. Check out what Rod Mollise has to say about buying a used SCT on his website, members.aol.com/rmollise Rod is the Guru on SCTs and he literally wrote the book, Choosing and Using the SCT....
3. Coatings have also steadly improved over the years.
4. The mounts, drives etc have steadly improved over the years, those oldies, not so fun to use....
5. Since this is the beginners forum, I am assuming this is your first scope. If so, I hope you have tried out some various SCTs and understand the differences in the mounts and the various problems associated with them...
Otherwise, a trip to a star party or club outing is in order...
jon
Posted 12/05/2004 11:48AM
#3
"How long coatings last" depends both on the quality
of the coating, and how they are treated. If they
are kept dry and free of contaminants, and are not
subject to poor cleaning technique, most coatings will
last almost forever.
That said, I have occasionally bought used stuff
where the coatings were degraded. In some cases
I'm sure it was a poor coating to start with (for
example inside an eyepiece), and in other cases I can
see evidence of poor cleaning (sleeks, abrasions).
Someone mentioned that professional observatories re-coat
every 2 or 3 years, which is true. But these scopes are
used maybe 3000 hours/year. Typical amateur might get
100 or 200 hours/year. Also, in an SCT the mirrors are
very protected, which helps also.
If you do find a 10 or 20 year-old scope with
excellent coatings, I think you can rest assured they will
still be excellent in another 10 or 20 if you take good
care of them. They have obviously passed the "test of time"
and were not defective to start with.
Things to watch out for are in lenses / correctors
are coatings with a strong haze or lines/patches with
missing coatings. A few pinpoint spots with missing
coating are not a big deal. Same goes for mirrors;
a little dust is fine, but discolored patchs are bad.
Posted 12/05/2004 01:25PM
#4
Last year I bought a 30+ year old C-8 from a fella in California who claimed the optics were pristine. When it arrived I was pleased to see that the scope, mirror and corrector all looked brand new. This one is so old it is the "holes in the forks" model.
I also have a C-90 that I bought new in the mid seventies that still looks new.
I also have a C-90 that I bought new in the mid seventies that still looks new.
Posted 12/05/2004 01:52PM
#5
Paul,
Coatings last a long time. As far as I know, anti-reflection coatings last forever. I've not heard much discussion, at any rate, about them fading or needing replacement. On the other hand, for the last many years, manufacturers have been "improving" these anti-reflection multi-coatings found on corrector plates, lenses, and eyepieces. Therefore, while the coatings on an old scope may be as good as new, the quality of their "new" would not match the current state of the art. What will happen in twenty years? I don't know. It could be that anti-reflection coatings have just about reached perfection, and there is not that much improvement left.
On the other hand, coatings on a mirror do deteriorate with time. (In protected SCT's probably less than in open newts's, and in scopes used at the ocean more than in scopes used in the desert.) How much they deteriorate depends on so many things that, in general, you should not buy a mirror depending on what condition it "might" be in, but what it "is" in. I've seen twenty year old mirrors in fine shape. I've seen ten year olds needing recoating. I haven't seen anything needing work in less than ten--although I have heard stories.
Alex
Coatings last a long time. As far as I know, anti-reflection coatings last forever. I've not heard much discussion, at any rate, about them fading or needing replacement. On the other hand, for the last many years, manufacturers have been "improving" these anti-reflection multi-coatings found on corrector plates, lenses, and eyepieces. Therefore, while the coatings on an old scope may be as good as new, the quality of their "new" would not match the current state of the art. What will happen in twenty years? I don't know. It could be that anti-reflection coatings have just about reached perfection, and there is not that much improvement left.
On the other hand, coatings on a mirror do deteriorate with time. (In protected SCT's probably less than in open newts's, and in scopes used at the ocean more than in scopes used in the desert.) How much they deteriorate depends on so many things that, in general, you should not buy a mirror depending on what condition it "might" be in, but what it "is" in. I've seen twenty year old mirrors in fine shape. I've seen ten year olds needing recoating. I haven't seen anything needing work in less than ten--although I have heard stories.
Alex
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